Low-noise spectroscopic ellipsometer

ABSTRACT

A spectroscopic ellipsometer comprising a light source ( 1 ) emitting a light beam, a polarizer ( 2 ) placed on the path of the light beam emitted by the light source, a sample support ( 9 ) receiving the light beam output from the polarizer, a polarization analyzer ( 3 ) for passing the beam reflected by the sample to be analyzed, a detection assembly which receives the beam from the analyzer and which comprises a monochromator ( 5 ) and a photodetector ( 4 ), and signal processor means ( 6 ) for processing the signal output from said detection assembly, and including counting electronics ( 13 ). Cooling means ( 12 ) keep the detection assembly at a temperature below ambient temperature, thereby minimizing detector noise so as to remain permanently under minimum photon noise conditions. It is shown that the optimum condition for ellipsometric measurement is obtained by minimizing all of the sources of noise (lamps, detection, ambient).

This is a non-provisional application claiming the benefit ofInternational application number PCT/FR01/01781 filed Jun. 8, 2001.

GENERAL FIELD AND STATE OF THE ART Background

The principles of a known state of the art ellipsometer are shown inFIG. 1.

Such an ellipsometer conventionally comprises a light source 1, apolarizer 2, an analyzer 3, and a detector 4 associated with amonochromator 5.

Those various elements are placed in such a manner that light output bythe source 1 passes thorough the polarizer 2 before reaching a sample Eto be analyzed, and then after being reflected on the sample E, itpasses through the analyzer 3 prior to reaching the detector 4 afterpassing through the monochromator 5 which is generally aphotomultiplier.

One of the two elements constituted by the polarizer 2 and the analyzer3 is a rotary element.

The output from the detector 4 is connected to processor means 6 whichperform Fourier analysis on the modulated signal as measured by thedetector 4 in order to determine information relating to the surfacestate of the sample E.

It is recalled that when light is reflected on a sample E, itspolarization is modified and that an ellipsometer setup makes itpossible to measure firstly the phase difference Δ and secondly theratio tan (Ψ) between the parallel and perpendicular polarizationcomponents of the beam as reflected on the sample.

By means of the monochromator 5, it is possible to perform measurementsat different wavelengths, thereby characterizing the optical propertiesof the material.

For a general presentation of spectroscopic ellipsometric techniques,reference can advantageously be made to U.S. Pat. No. 5,329,357 (Bernouxet al.) which relates specifically to the advantage of adding opticalfibers to the setup.

The visible spectroscopic ellipsometers available on the marketgenerally operate in a spectral range of 1 micrometer (μm) to 230nanometers (nm), using a xenon arc source (selected for high radiantflux density or “irradiance”).

Nevertheless, ellipsometers have been proposed that are capable ofoperating over a broader spectral range than the above-mentionedellipsometers and that include an additional source, such as a deuterium(D₂) source that provides less of a point source, emitting in the range130 nm to 700 nm at a power of 30 watts (W) to a few hundreds of wattsor more.

The detectors that are used are generally detectors of the Si or Gephotodiode type or photomultipliers (generally multi-alkaliphotomultipliers), operating at ambient temperature.

They use very high quality optical systems, possessing polarizationextinction coefficients of about 10⁻⁵, and very high transparency, evenin the ultraviolet.

This makes it possible in the above-specified spectral range todetermine the Ψ and Δ coefficients with precision equal to or less than1/1000^(th) of a degree (°).

Furthermore, the processor means of most ellipsometers implement asimplified photon counting method, which method is known as the“Hadamard method”. That method consists in counting photons with asignal that is amplitude sampled over a very limited number of channels:eight counters or channels, for each period of rotation of the rotaryelement of the ellipsometer (a configuration with a rotary polarizer oranalyzer (modulated polarization) and/or a rotating plate (phasemodulation)).

Drawbacks of State of the Art Ellipsometers

Ellipsometers of the type described above present several limitations.

A first limitation is associated directly with fluctuations in thesource, i.e. with its lack of stability, with this constraint beingknown as shot noise limitation (SNL).

Another limitation is associated with noise coming from ambient lightand also referred to as “leakage noise”, which can in theory beeliminated by isolating the entire ellipsometer (and not only thephotomultiplier) completely from ambient light, but which neverthelessremains a difficulty encountered by many ellipsometer manufacturers.

Another limitation lies in the dark current or intrinsic noiseassociated with the photomultiplier and its internal amplificationsystem. This noise is commonly referred to as detector noise limitation(DNL). It should be observed that all of the frequencies correspondingto the bandwidth of the photomultiplier are generally present therein.

Thus, the Hadamard sums (as determined over quarter periods of themodulated signal) are calculated by taking account of a previouslymeasured offset which corresponds to the leakage noise and to the DNL.

Nevertheless, although conventional ellipsometers correspond in practiceto synchronous detection (in-phase frequency filtering of the signalmodulation), the Hadamart method becomes difficult when the amplitude ofthe modulation is low.

For a signal modulated at Ω, the amplitude of the spectrum component at2Ω in the signal is of the same order of magnitude as the amplitude ofthe noise (with this being true more particularly in the ultravioletwhere counts of only 100 to 1000 counts per second (cps) are measured).

The signal components are thus “buried” in the noise level which itselfcorresponds to a superposition of the spectrum density of the sourcenoise, shot noise when using a xenon arc, ambient light, and noise fromthe detector and its associated electronics.

Furthermore, with conventional ellipsometers, when it is desired to workat wavelengths shorter than 200 nm, the observed signal/noise ratio isunfavorable.

The only known way of eliminating the effects of the various sources ofnoise is to increase acquisition times.

Unfortunately, measurement is then subject to systematic error, inparticular concerning wavelengths shorter than 200 nm. This means thatequipment must be pre-calibrated in use.

Furthermore, it should also be observed that another problem encounteredwith ellipsometers that use additional sources to enlarge theiroperating range is the problem of their cost and of the power that mustbe supplied to them.

Under such conditions, it is practically impossible to envisage a systemthat is sufficiently compact for in situ measurement (integratedmetrology) even in a photon-counting system as described above. Theimpossibility of having a measurement head internal to the metrologicalcasing leads to a limitation due to the windows of the casing givingrise to birefringent effects that need to be corrected.

SUMMARY OF THE INVENTION

An object of the invention is to mitigate those drawbacks.

In particular, the invention provides an ellipsometer structure in whichnoise is minimized.

Techniques are known, in particular from the abstract of Japanese patentapplication No. 0907995, that consist in cooling photomultipliers inapplications that are very different from ellipsometer applications.

Those cooling techniques are not intended in any way to reduce noise.They serve to lower detection limits as much as possible.

The invention proposes a spectroscopic ellipsometer comprising a lightsource emitting a light beam, a polarizer placed on the path of thelight beam emitted by the light is source, a sample support receivingthe light beam output from the polarizer, a polarization analyzer forpassing the beam reflected by the sample to be analyzed, a detectionassembly which receives the beam from the analyzer and which comprises amonochromator and a photodetector, and signal processor means forprocessing the signal output from said detection assembly, and includingcounting electronics.

This ellipsometer presents the characteristic of comprising coolingmeans for keeping the detection assembly at a temperature lower thanambient temperature, in particular at a temperature of about −15° C., orlower.

Also advantageously, its source is a deuterium lamp preferably having apower of about 30 watts.

Other low noise sources can be envisaged, and in particular plasma lampand halogen lamp sources.

Also advantageously, the counting electronics is suitable for performingamplitude sampling over a number of channels lying in the range 8(Hadamard equivalent) up to 1024 (filtered Fourier), and particularlypreferably about 1000 or more, in particular a number of channels lyingin the range 1024 to 8192 (depending on the type of encoder).

The processing means implement Fourier analysis on the signals sampledin this way.

Thus, the proposed ellipsometer enables noise to be minimized (toimprove its precision): i) with total protection from ambient light; ii)no polluting environment (mechanical vibration and/or sources ofelectromagnetic noise); and a detector operating by counting photons ina minimum intrinsic noise level which is obtained in this case bycooling (12) to keep the detection assembly at a temperature lower thanambient temperature. By providing better performance in terms of signaldetection, it makes it possible to use optical fibers all the way to 160nm. This makes it possible to operate in compact manner in the contextof integrated metrology associated with current development of clustertools in the field of thin layer technology. The system becomes fullyintegrated in the in situ casing since it makes it possible to usefilms.

BRIEF DESCRIPTION OF THE FIGURES

Other characteristics and advantages of the invention appear furtherfrom the following description which is purely illustrative andnon-limiting and should be read with reference to the accompanyingdrawings, in which:

FIG. 1, described above, is a diagram illustrating the principles of aspectroscopic ellipsometer known in the state of the art;

FIG. 2 is a block diagram of an ellipsometer constituting an embodimentof the invention;

FIGS. 3a and 3b are graphs on which measurements of the parameters Ψ andΔ are plotted as a function of wavelength for an ellipsometer as shownin FIG. 2 and for a standard ellipsometer; and

FIG. 4 is a graph showing direct trace measurements obtained by anellipsometer as shown in FIG. 2 and by a standard ellipsometer, themeasurements being plotted as a function of wavelength.

DESCRIPTION OF POSSIBLE EMBODIMENTS OF THE INVENTION Background

A spectroscopic ellipsometer constituting a possible embodiment of theinvention presents the following characteristics.

1) Its source 1 is a low noise source, i.e. a source whose frequencydispersion is much less than that of a xenon arc lamp.

Such a source is advantageously a deuterium D₂ lamp.

Deuterium D₂ lamps are lamps having particularly low noise. They aremuch more stable than xenon arc lamps (stability in a ratio of 20), andthey also provide much better stability than other types of lamp in theultraviolet and in a portion of the visible.

Other types of lamps could be envisaged. In particular halogen lamps(towards the infrared) or plasma discharge lamps (visible and UV), orplasma sources are particularly suitable, even when they have emissionspectrum lines (non-continuous spectrum).

2) Its photomultiplier 5 is placed together with magnetic protection ina low temperature environment, thereby reducing and stabilizing its darkcurrent.

For this purpose, a Peltier effect cooling system is used (−15° C.).

By reducing the dark current of the photomultiplier, noise is reduced byat one least decade (from 200 cps to 10 cps, for example).

Thus, the above-mentioned effects of offset instability are eliminated.

Temperatures that are even lower further improve the performance of thephotomultiplier to a few counts per second.

In a spectral range such as the ultraviolet, when using a D₂ lamp,source emission noise is low. DNL is then of the same order of magnitudeas the SNL limit. This amounts to saying that the number of dark currentcounts N_(d) continues to be negligible, and the signal is degradedsolely by a low level of residual source noise N_(ph) (number of photonsdue to source emission). Because of the intrinsic dark current of thephotomultiplier is reduced, precise ellipsometic measurement isobtained, which is not possible when N_(ph)≦N_(d).

This reduction in dark current makes it possible in particular toperform measurements at 187 nm, with a low power lamp and in anenvironment that is not restricted (no vacuum, no purging with an inertgas, no dry atmosphere). The photomultiplier operates in a photoncounting mode and ideally it is linear (no non-linearity associated withavalanche overlap effects (saturation in analog mode)).

This limit of 160 nm can also be crossed by using inert gas conditions(nitrogen) with a PMT (R7639 from Hamamatsu) and a correspondingmonochromator. In the extreme ultraviolet (120 nm to 160 nm), it isadvantageous to cool the base of the photomultiplier 5, or to perform N₂sweeping in the photomultiplier enclosure cooled by a cooler element ofthe Peltier heat exchanger type, thus avoiding the need for a MgF₂window.

3) Furthermore, the proposed spectroscopic ellipsometer counts over alarge number of channels: up to more than 1024 channels, which combinedwith Fourier analysis oversamples the signal and provides effectivefiltering of high frequency noise components, i.e. of intrinsic noisefrom the sources and the detector and the electronics.

The time required for counting is reduced.

This counting technique turns out to be much better than that which ispossible using the Hadamard method. It can be implemented very simply byusing commercially available electronics. The oversampling it providescontributes to attenuating noise at all of the high frequenciesassociated with lamp noise.

Detailed Example of One Possible Embodiment

An example of an ellipsometer constituting a possible embodiment isdescribed below in detailed manner with reference to FIG. 2.

This ellipsometer is of the type having a rotary polarizer.

It operates in the range 180 nm to 750 nm. It can be used in a vacuum orin a controlled atmosphere so as to extend its operating range to therange 130 nm to 720 nm.

The source 1 is a deuterium lamp (D₂) having power of 30 W and a pointsource diameter of 0.5 millimeters (mm) (Oriel 63163 lamp or HamamatsuL7295 or L7296 lamp).

The light beam is transferred from the source 1 by means of a 600 μmsingle strand fused silica fiber 7 to the rotary polarizer 2. One of thefunctions of the fiber is specifically to eliminate the residualbirefringence of the source 1.

Coupling with the rotary polarizer 2 is performed via a convergingelement 8 of fused silica, selected for its low residual birefringenceand its transparency in the ultraviolet.

It can also be implemented using an assembly comprising a concave mirrorand a plane mirror, both having MgF₂ surface treatment.

The sample E for analysis is placed at the outlet from the polarizer 2on a support 9 whose orientation can be adjusted.

The beam reflected by the sample is applied to the analyzer 3.

Both the polarizer 2 and the analyzer 3 are made of MgF₂ (for examplethey are constituted by analyzers and polarizers from Fichou/Optiquewhich certifies 2.5° of deviation at 250 nm and a passband to 10electron volts (eV)). This choice of material makes it possible toobtain greater transparency in the ultraviolet.

The polarizer 2 is rotated at a frequency of about 10 Hz (with thecriteria for selection being associated with the environment, mainsfrequency or vibration frequency) and is controlled by a mechanicalassembly of the stepper type (microstep).

After being reflected on the sample and passing through the analyzer 3,the beam is refocused by a set of mirrors 11 and is applied to the inletof the monochromator 5 which is a dual monochromator having an Oriel77250 type ⅛ M grating blazed at 250 nm with 1200 lines (180 nm to 500nm in first order and an intermediate 0.6 mm slot; its resolution at 500nm is 4 nm). Gratings blazed at 200 nm but with 600 lines per millimeter(mm) can be used.

While performing a measurement, the system automatically incorporatestwo filters in succession so as to eliminate higher diffraction ordersfrom the gratings of the monochromator. Control is performed by means ofan Oriel filter passer and an Ni DAQ (TTL) interface from NationalInstrument.

The output from the monochromator 5 is applied to the detector 4 whichserves to count photons. The detector 4 is of the tube type and it issold by Hamatsu under the reference R2949 or R7639.

The detector 4 is placed in a cooler 12 of the C-659S type whichmaintains it at a temperature of −15° C.

The counting electronics includes a discriminator 13 connected to thedetector 4. The discriminator is of the type sold by Hamatsu under thereference C 3866 and it has a linear dynamic range of 10⁷.

The detector 4 and the discriminator 13 are selected for their low darkcurrent characteristics (159 cps at 25° C. and dropping to less than 10cps when cooled for the R2949 and to less than 1 cps for the R7639(which has quantum efficiency of 44% at 160 nm)). This can beimplemented using water cooling or “cryogenic” nitrogen flow coolingwith external cooling being provided by Peltier cooling elements and anexternal heat exchanger. The detector and the discriminator are alsoselected for their sensitivity in the blue of 8.3 μA/1 m with gain of10⁷. The photon counting electronics is linear for 10⁷ photons.

The TTL output from the discriminator 13 is analyzed by means of amultiscale count card 14 (MCS II Nuclear Instrument or FMS CanberraElectronics card CM 7882) capable of analyzing 8192 count channels, withtwo simultaneous inputs and a sampling time of 2 μs.

The card 14 is controlled by a computer 15 operating in a Windows NTserver environment with object C++ programming coupled with commercialactive X components, in this case the Works++ components from NationalInstruments.

Example of Results

FIGS. 3a and 3b show measurements of Ψ and Δ obtained over a wavelengthrange of 1 nanometer (around 250 nm) respectively when using a standardxenon lamp ellipsometer, photomultiplier at ambient temperature andHadamart detection, and when using an ellipsometer as described above.

It can be seen that measurements are much more widely dispersed with thestandard ellipsometer than with an ellipsometer of the type describedabove.

Direct traces obtained with each of the two ellipsometers have also beencompared.

This is shown in FIG. 4.

The improvement is also clearly visible.

It is recalled that for a direct trace, it is necessary that tan ω=CosΔ=1.

This turns out to be exactly true (ignoring optical alignment) for theabove-described low noise spectroscopic ellipsometer, whereas with astandard ellipsometer, the difference is much larger for Cos Δ whichmeans that it is then important to normalize α and β.

One application consists in extending an in situ setup. The rotarypolarizer system and the analyzer are MgF₂ lumps mounted in a vacuum onstepper micromotors (vacuum technology) having a hollow shaft (in whichthe MgF₂ lump is inserted) and the optical encoder which can thus bepositioned even inside a casing or a cooling and measurement chamber ofa cluster tool type reactor. The source and analysis inputs are thencompact blocks. This makes it possible to implement two heads (analyzerand polarizer being equivalent). An estimate of the physical size thatcan be achieved corresponds to a cylinder having a diameter of about 40mm and a length of 60 mm to 70 mm. Windows which are sources ofbirefringence and of absorption are thus eliminated since only theoptical fibers are connected to the casing of the reactor. It has beenshown that such a system can operate in situ for photons havingwavelengths in the spectrum range 160 nm to 170 nm.

1. A spectroscopic ellipsometer comprising a light source (1) emitting alight beam, a polarizer (2) placed on the path of the light beam emittedby the light source, a sample support (9) receiving the light beamoutput from the polarizer, a polarization analyzer (3) for passing thebeam reflected by the sample to be analyzed, a detection assembly whichreceives the beam from the analyzer and which comprises a monochromator(5) and a photodetector (4), and signal processor means (6) forprocessing the signal output from said detection assembly, and includingcounting electronics (13), the ellipsometer being characterized in thatit further comprises cooling means (12) for keeping the detectionassembly at a temperature lower than ambient temperature.
 2. Anellipsometer according to claim 1, characterized in that said coolingmeans (12) are suitable for keeping the detection assembly at atemperature of about −15° C. or lower.
 3. An ellipsometer according toclaim 1, characterized in that the source (1) is constituted by adeuterium lamp.
 4. An ellipsometer according to claim 3, characterizedin that the lamp has power of about 30 watts.
 5. An ellipsometeraccording to claim 1 or claim 2, characterized in that the source isconstituted by a cold plasma lamp.
 6. An ellipsometer according to claim1 or claim 2, characterized in that the source is constituted by ahalogen lamp.
 7. An ellipsometer according to claim 1, characterized inthat the counting electronics (13) is suitable for performing amplitudesampling over a number of channels equal to about 1000 or more.
 8. Anellipsometer according to claim 6, characterized in that the countingelectronics (13) is suitable for implementing amplitude sampling over anumber of channels lying in the range 1024 to
 8192. 9. An ellipsometeraccording to claim 7 or claim 8, characterized in that the processormeans (6) apply Fourier analysis to the signal output by the countingelectronics.
 10. A spectroscopic ellipsometer comprising: a light sourceconfigured to emit a light beam, a polarizer placed on the path of thelight beam emitted by the light source, a sample support configured tosupport a sample such that the sample is arranged to receive the lightbeam output from the polarizer, a polarization analyzer arranged toallow passage of the light beam reflected by the sample, a detectionassembly including a monochromator arranged to receive the light beamfrom the analyzer and a photodetector arranged to receive an output fromthe monochromator, and a signal processor configured to process a signaloutput from said detection assembly, wherein the ellipsometer furthercomprises a cooling apparatus configured to maintain the monochromatorand the photodetector at a temperature lower than ambient temperature.11. The ellipsometer as recited in claim 10, wherein said coolingapparatus is configured to maintain the detection assembly at atemperature of about −15° C. or lower.
 12. The ellipsometer as recitedin claim 10, wherein the light source is a deuterium lamp.
 13. Theellipsometer as recited in claim 12, wherein the lamp has power of about30 watts.
 14. The ellipsometer as recited in claim 10, wherein the lightsource is a cold plasma lamp.
 15. The ellipsometer as recited in claim10, wherein the light source is a halogen lamp.
 16. The ellipsometer asrecited in claim 10, wherein the signal processor includes countingelectronics, and wherein the counting electronics are configured toperform amplitude sampling over a number of channels equal to about 1000or more.
 17. The ellipsometer as recited in claim 16, wherein thecounting electronics are configured to implement amplitude sampling overa number of channels lying in the range 1024 to
 8192. 18. Theellipsometer as recited in claim 16, wherein the signal processor isconfigured to apply Fourier analysis to a signal output by the countingelectronics.
 19. An ellipsometer comprising: a detection assemblyincluding a monochromator arranged to receive a beam of light and aphotodetector arranged to receive an output from the monochromator andconfigured to count photons in the output received from themonochromator; a cooling apparatus, wherein the cooling apparatus isconfigured to maintain the detection assembly at a temperature lowerthan ambient temperature.
 20. The ellipsometer as recited in claim 19,wherein the cooling apparatus is configured to maintain the detectionassembly at a temperature of approximately −15° C. or lower.
 21. Theellipsometer as recited in claim 19, further comprising a light sourceconfigured to emit the beam of light.
 22. The ellipsometer as recited inclaim 21, further comprising a polarizer configured to polarize the beamof light.
 23. The ellipsometer as recited in claim 22, wherein thepolarized beam of light is reflected from a sample material through apolarization analyzer to be received by said detection assembly.
 24. Theellipsometer as recited in claim 23, wherein said ellipsometer furthercomprises a pair of mirrors arranged to focus the beam of light to bereceived by said detection assembly to an inlet of the monochromator.25. The ellipsometer as recited in claim 24, wherein the photodetectoris configured to count photons.
 26. A method of reducing detector noisein an ellipsometer, the method comprising: cooling a detection assemblywithin the ellipsometer so as to maintain the detection assembly at atemperature lower than ambient temperature, the detection assemblyincluding a monochromator and a photodetector coupled to receive anoutput from the monochromator; and analyzing optical properties of asample material using an output signal of the detection assembly of theellipsometer.
 27. The method as recited in claim 26 further comprisingmaintaining the detection assembly at or below a predeterminedtemperature.
 28. The method as recited in claim 27, wherein thepredetermined temperature is −15° C.
 29. The method as recited in claim26, further comprising: projecting a beam of light from a light source;reflecting the projected beam of light off the sample material; themonochromator receiving the reflected beam of light.
 30. The method asrecited in claim 29, further comprising polarizing the beam of lightprior to said reflecting.
 31. The method as recited in claim 30, furthercomprising using mirrors to focus the reflected beam of light prior tosaid receiving.